Method and apparatus for the production of seismic shocks

ABSTRACT

A releasable heavy mass is provided which is capable of producing by impact with the ground a shock which can be used in seismic prospecting. 
     This mass is characterized by the fact that it comprises very many individual heavy elements connected together in a way loose enough to allow deformation of the mass without loss of cohesion on impact, and in that its base is of considerable area relative to the surface of the whole mass and is maintained substantially plane at least after release and up to impact. The mass is, in seismic prospecting, released from a helicopter.

BACKGROUND OF THE INVENTION

The present invention concerns the generating of shocks in the groundfor seismic exploration, and in particular concerns the production ofshocks by the dropping of a weight on to the ground.

The weight dropping method has been known for a long time and isparticularly described in the United States of America patent No.2,851,121. In this known method a lorry fitted with a lifting jib isused to raise a heavy solid body to a certain height which is thenallowed to fall on to the ground. In current practice, for example, aheavy mass of three tons is dropped from a height of three meters.Usually the energy released in the ground by the shock produced by theimpact of the heavy mass is relatively low, and so it is advantageous touse a succession of such shocks giving rise to different, refracted orreflected waves which return through the ground to seismic detectors,and to integrate the different signals corresponding to different shocksto obtain a composite signal containing significant data.

The method of prospecting by weight dropping, described above, offersthe essential advantage of being able to be carried out near houses(which is not the case with methods involving the exploding of charges);it is not in itself very effective from the seismic point of view, inthe sense that it only introduces low amounts of energy into the ground;in any case, it requires the use of a heavy land vehicle, whichprecludes its use in hilly areas.

It is an object of the present invention to provide a new mode ofseismic prospecting by weight dropping particularly involving a newheavy mass.

Another object of the present invention consists in the replacing of theland vehicle previously used by an aircraft, particularly a balloon or ahelicopter. Helicopters are known to be more and more used in seismicexpeditions. A priori, the use of aircraft allows the release of a heavymass from a height much greater than usual heights and the potentialenergy of the heavy mass, which varies with the height of the drop, canbe especially increased thereby, all things staying equal in otherrespects. For example, it seems advantageous, from the energy point ofview, to work with a mass of 500 kgs for dropping heights comprisedbetween 5 and 100 meters.

In a complementary way, the increase in the dropping height increasingthe speed of arrival on the ground results in an appreciable enlargementof the spectrum of the seismic wave produced, which improves the seismicdata obtained.

The first trials conducted by applicants consisted purely and simply intransposing the conventional technique to the release-by-aircraftsituation, using a hovering helicopter equipped for transporting bysling and fitted with a system of electromagnetic release controlled bythe pilot. Heavy masses, solid and rigid, of various shapes werereleased thus from various heights.

The results obtained proved nearly unusable for all of the followingreasons:

on hard terrain and even on relatively loose terrain, the mass reboundson its first impact with the ground and gives rise to a series ofsuccessive shocks in a random and unpredictable way; for that reason,the reflected or refracted seismic waves are not exploitable inpractice: they contain parasite waves.

on very loose terrain, the mass buries itself in the ground, and thereis a barely usable shock wave, and a significant deterioration in thesurface of the ground.

Thus, it is apparent that the method putting into practice a weight dropfrom an aircraft cannot be a simple transposition of known weightdropping methods.

Essentially, for a seismic prospecting by aircraft, the applicantspropose a new releasable heavy mass characterised by its structure andby its functions.

SUMMARY OF THE INVENTION

The releasable heavy mass according to the invention is essentiallycharacterised in that it comprises very many individual heavy unitsconnected together in a way sufficiently loose to allow the deformationof the mass without the assembly, however, losing its unity on impact.Furthermore, the mass comprises, according to the invention, means forensuring and maintaining the inherent flatness of the area of themovable mass coming into contact with the ground. A seismic prospectingmethod using this mass involves releasing the mass from an aircraftwhich preferably is a helicopter.

In a first embodiment, the individual heavy elements can be formed bypieces of large metal shot connected by a special casing. In a secondembodiment, preferred at the moment, taking into account realisedexperimental tests, the individual elements are formed by linksconnected together to form segments of chain disposed in the form of astar or in the form of a cobweb pattern, by elastically deformable,jointed structures.

The specification which follows with reference to the attached drawingswill make the invention clearer, while making evident all its advantagesand characteristics with reference to preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents diagrammatically a heavy mass according to theinvention in position about to be released.

FIG. 2 represents this same mass at the time of impact with the ground,

FIG. 3 represents diagrammatically a casing of this mass,

FIG. 4 represents diagrammatically the cobweb of chains forming thepreferred embodiment,

FIG. 5 is a section along the section line V--V of FIG. 4,

FIG. 6 shows a way of suspending the cobweb of chains.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First the embodiment in FIGS. 1, 2 and 3 will be described. A heavy mass100 is formed of metal shot, particularly of iron shot, enclosed inseveral casings 101, 102 . . . . The individual elements of shot have,for example, a diameter of 0.5 to 3 cm. The exterior casings 101, 102 .. . must be both resistant and flexible. An alternation of resistant andflexible casings can be used. The resistant casings are advantageouslyformed by a metal trellis made of cables or of metal strands;preferably, metal strands made from thin flexible strips, as representedin FIG. 3, are used. The flexible casings are advantageously made ofrubberized fabric. As a variant, a superposition of metal nets ortrellis can be used. A single casing of rubber strengthened with wirecan also give satisfaction.

In the suspended state shown in FIG. 1, the mass 100 takes, asrepresented, the shape of a pear. At the base of this pear, a planeframework 103 is placed, rigid enough to inherently give the base atleast approximate flatness, as shown. The centre of gravity of thesuspended mass is nearer the base 103 than the suspension point 110.

FIG. 2 shows impact of the mass 100 on a relatively hilly piece ofground S. The flexibility of the mass allows it to be deformed on impactto assume the shape of the ground S and to absorb, if not to suppresscompletely, the rebound effect. Moreover, the width of the base 103substantially avoids penetration by the mass when it falls on to verysoft ground. Finally, the inherent flatness of the base 103 ensures, atthe time of an impact on relatively flat ground, the brevity of theimpact, all points of the base, in practice, coming into contact withthe soil together.

Of course, the suspension and release apparatuses for the mass 100comprise means known in themselves and suitable for seismic applicationand thus require no further description. For detecting arrival on theground in particular, the mass of shot incorporates a shock detector,for example of the known type in which the shock closes an electriccontact; such a detector is advantageously connected by an electriccable to the release helicopter, this electric cable being, beforerelease, wound on a roller with an automatic resilient return.

FIGS. 4 and 5 illustrate the most preferred embodiment of the invention.The mass 200 according to this preferred embodiment comprises a centralmetal disc 210 on which is mounted, as before, a shock detector oraccelerometer 220. Around the central axis of the disc 210 extend sixradial arms 230. As can be better seen in FIG. 5, each arm 230 is formedby an element of metal cable 231 fixed at one end 231a to the disc 210and held at its other end 231b by a keeper or holding element. On eacharm 230 are threaded successively a series of springs 232 and hollowmetal braces 233, each brace having, as seen in the drawings, the shapeof a spool. The springs 232 are put under compression and all theelements of each arm are thus maintained in radial tension under theeffect of the springs. Each arm 230 thus forms a jointed assembly whichis deformable but which always returns to the straight position.

Chains 251, 252, 253, 254, are mounted in a structure of concentrichexagons at different distances from the central disc 210. To do this,the angle links of the chains span the different braces. Thus a flexiblestructure is finally obtained, with elements of individual linksextending substantially in a plane according to a network of chains anddeformable arms, in the form of a cobweb. In a typical example, thecentral disc can have a weight of the order of 100 kg for a chain weightof 350 kg and a total weight of the order of 500 kg.

It is moreover advantageous to provide an interchangeable central disc210 in order to be able to adapt the releasable mass 210 to differentconditions of use by slight variations of the distribution of themasses.

It is easily seen that the releasable mass 200 possesses, to a greaterdegree even than the mass 100, the properties which have appeared fromexperiment to be necessary for a weight drop carried out from a greatheight which is to be usable seismically, namely, the properties offlexibility, large base surface and inherent flatness.

Of course, other geometric arrangements than the hexagon can be realisedand are included in the invention. Numerous networks of chains can beused placed in a symmetrical fashion around a disc or a central weight,with resilient means tending to maintain the inherent flatness of thestructure in a polygonal or circular arrangement.

For such plane structures of chains, particular precautions must betaken for suspension and release, and FIG. 6 shows a preferred mode ofsuspension for the mass 200 which is represented schematically byoutlines. Six suspension slings 250 hold the mass 200 at the angles ofits external periphery and one at its centre; these seven slings arejoined at 260, another sling 265 joining their connection point 260 tothe point of suspension 270 from a helicopter. It must be noted that inthis suspension condition, the mass is slightly deformed, its concavitybeing turned upwards. On release, the springs 232 bring the mass 200back to inherent flatness.

Again, after release, the mass must fall without the slings 250 comingbetween the mass and the ground. To do this, the slings are fitted withbrace-sails 280 made, for example, of fabric, which are capable ofcreating a drag (parachute effect) to keep the point 260 absolutelyabove the mass 200. In all cases where it is desired that the mass 200keep rigorously horizontal, the brace-sails 280 can also be given a sizesufficient for the parachute effect to brake the drop of the massslightly while keeping the latter horizontal.

Thus the deformable mass according to the invention enables theproduction of seismic shocks by weight dropping from aircraft withconsiderable drop heights and particularly drop heights of between 5 and100 meters, the shock waves obtained being most often more exploitableand richer in data than those conventionally obtained.

In a simplified variant of the apparatus of FIGS. 4 and 5, the radialarms, anchored in the central disc 210, are cables exhibiting a rigiditysufficient to ensure, once the mass has been released, to restoration ofthe mat of chains to the plane condition. On each arm are threadedsuccessively a series of braces whose role is to maintain the distancebetween the concentric hexagons of the chain.

In another variant of embodiment, the jointed arms are formed by cableson which are threaded successively cylinders of rubber or similarcompressible material and round links forming part of chain elements,such as 251, 252, 253, 254.

Such cylinders of rubber are kept under compression and play the samerole as the springs 232, in such a way that each elastically deformablearm is maintained in a state of radial tension while extended.

I claim:
 1. A method for producing shocks in the ground, comprising thestep of releasing a mass for free fall from an aircraft from a height ofbetween about 5 and 100 meters, said mass comprising a multiplicity ofindividual heavy elements connected together, and a base of largerelative surface, said mass on impact with the ground deforming withoutlosing cohesion, said release step including the steps of: maintainingsaid base substantially plane by providing means for stiffening thebase; and maintaining the base substantially horizontal at least afterrelease and up to impact.
 2. A releasably heavy mass capable of beingreleased from an aircraft from a height of between about 5 and 100meters and of producing by impact with the ground a shock exploitable inseismic prospecting, said mass comprising:a multiplicity of individualheavy elements connected together in a way loose enough to allowdeformation of the mass without loss of cohesion on impact, the masshaving a lower portion comprising a base portion; and base stiffeningmeans for stiffening said base portion of the mass and biasing at leastthe lower of said elements towards a substantially horizontal base planeat least after said release and up to said impact.
 3. A releasable heavymass capable of being released from an aircraft from a height of betweenabout 5 and 100 meters and of producing by impact with the ground ashock exploitable in seismic prospecting, said mass comprising:amultiplicity of individual heavy elements connected together in a wayloose enough to allow deformation of the mass without loss of cohesionon impact; and base stiffening means for biasing at least the lower ofsaid elements towards a substantially horizontal base plane at leastafter said release and up to said impact, wherein said heavy elementscomprise shot surrounded by at least one flexible and resistant casingand said base stiffening means comprises at least one framework with asubstantially planar base.